1. Field of the Invention
The present invention relates to a Zener diode and more particularly, to a low-noise Zener diode applicable to voltage reference, which provides an improved surge resistance performance.
2. Description of the Prior Art
A conventional low-noise Zener diode is shown in FIG. 1, in which a p-type diffusion region 22 with a circular plan shape is formed in a surface area of an n-type semiconductor substrate 21. The diffusion region 22 serves as a guard ring.
A p.sup.+ -type diffusion region 23 is formed in the surface area of the substrate 21 so as to overlap with the inner part of the guard ring 22. The diffusion region 23 is positioned within an outer boundary line 22a of the guard ring 22. The central bottom face of the diffusion region 23, which is positioned within an inner boundary line 22b of the guard ring 22, is exposed from the guard ring 22, producing a p-n junction 27 at the interface of the region 23 and the substrate 21. The p-n junction 27 has a circular plan shape and a specified reverse breakdown voltage.
A silicon dioxide (SiO.sub.2) film 24 having a window 24a is formed on the surface of the substrate 21. The window 24a is positioned on the p.sup.+ -type diffusion region 23 so as to expose the top face of the region 23.
An anode electrode 25 is formed so as to be in contact with the p.sup.+ -type diffusion region 23 through the window 24a. A cathode electrode 26 is formed on the back of the substrate 21.
In the conventional Zener diode shown in FIG. 1, a breakdown current flows through the p-n junction 27 due to its Zener breakdown during operation. As shown in FIG. 2, the level of noise generated from the diode decreases or increases with the increasing breakdown current density.
The noise level of the diode is comparatively low in the middle region B. Therefore, this region B is termed the "low-noise region".
The noise level increases abruptly with the decreasing breakdown current density in the region A whose breakdown current density value is lower than that of the region B. Such noise-level change is caused by shot noises which are due to the fact that the Zener breakdown phenomenon does not take place uniformly at the entire p-n junction 27. Therefore, the region A is termed the "shot noise region".
The noise level increases gradually with the increasing breakdown current density in the region C whose breakdown current density value is greater than that of the region B. Such noise-level change is caused by thermal noise, which is due to the heat generation at the p-n junction 27 because of the excessively high level of the breakdown current density. Therefore, the region C is termed the "thermal noise region".
The size or diameter D.sub.1 of the p-n junction 27 is designed in consideration of the noise characteristic of the diode shown in FIG. 2. Specifically, to ensure that the diode operates in the low-noise region B, in other words, to ensure that the breakdown phenomenon takes place uniformly over the entire p-n junction 27, when the breakdown current is within the practical value range (for example, several milliamperes), the diameter D1 is usually designed to be short such as 30 to 60 .mu.m.
In the conventional Zener diode shown in FIG. 1, since the p-n junction 27 is necessarily designed to be short for shot-noise reduction purposes or low-noise operation purposes, a problem that an obtainable surge resistance performance of the diode becomes lower than that of the popular voltage reference diodes occurs. Therefore, this conventional Zener diode cannot be employed for large surge applications in which a large current and/or voltage surge tends to occur.
To solve this problem, another conventional low-noise Zener diode shown in FIGS. 3 and 4 was developed, which was disclosed in the Japanese Non-Examined Patent Publication No. 62-43184 published in February 1987.
The conventional diode of FIGS. 3 and 4 is the same in structure as that of FIG. 1 except for resistor regions 28a and 28b. Each of the regions 28a and 28b is made of an n-type diffusion region and is the same in area or size as each other. Therefore, the description regarding to the same structure is omitted here by adding the same reference numerals to the corresponding elements for the sake of simplification.
As shown in FIGS. 3 and 4, the outer resistor regions 28a and the inner resistor regions 28b are formed in the p.sup.+ -type diffusion region 23 with circular plan shape. The outer resistor regions 28a are arranged at regular intervals along an outer circle concentric with the circular region 23. The inner resistor regions 28b are arranged at regular intervals along an inner circle concentric with the outer circle. The arrangement density or pitch of the outer resistor regions 28a is higher than that of the inner resistor regions 28b. In other words, the arrangement density or pitch of the resistor regions 28a and 28b increases with increasing distance from the center of the diffusion region 23.
With the conventional Zener diode of FIG. 1, breakdown current tends to flow only through the periphery of the p-n junction 27 and consequently, the heat generation tends to concentrate at the periphery of the junction 27. On the other hand, in the conventional Zener diode of FIGS. 3 and 4, because the resistor regions 28a and 28b are formed in the diffusion region 23, the breakdown current uniformly can flow through the entire p-n junction 27 even when the breakdown current is within the practical value range (for example, several milliamperes).
Accordingly, heat generation is dispersed over the entire p-n junction 27 in operation, which permits the the surge resistance performance to be improved.
However, the conventional Zener diode of FIGS. 3 and 4 has the following problem:
In the conventional Zener diode of FIGS. 3 and 4, similar to the conventional Zener diode of FIG. 1, the diameter D.sub.1 of the p-n junction 27 cannot be enlarged because such diameter enlargement causes increases in shot noises due to decreases in breakdown current density.
Therefore, if a large surge current and/or voltage is applied across the anode and cathode electrodes 24 and 26 so as to generate heat at the entire p-n junction 27, the conventional Zener diode of FIGS. 3 and 4 cannot withstand such surge current and/or voltage.